1. Field of the Invention
The present invention relates to a tetragonal zirconia polycrystalline (TZP) ceramic, and more particularly, to a method for producing a strong and aging-free TZP ceramic material by the formation of silica/zircon on the surface thereof.
2. Description of the Prior Art
TZP ceramics are one of the most widely used ceramic materials because of their excellent mechanical properties of hardness, strength, and fracture toughness. In spite of these merits, the TZP ceramics have two limitations; relatively low high-temperature strength and low-temperature environmental degradation.
The decrease in strength at high temperature is inherent to these materials because a martensitic transformation from tetragonal to monoclinic phase is the main toughening/strengthening mechanism. The driving force for the transformation decreases steadily with temperature until the monoclinic phase is thermodynamically stable, at which no phase transformation occurs.
Another drawback of the TZP ceramics is an unusual reduction in strength when exposed to air at temperatures between 150 and 400.degree. C. [1]. This phenomenon is generally referred to as low-temperature environmental degradation, or just aging of the TZP. It is well known that the aging occurs by the spontaneous transformation of the zirconia from tetragonal to monoclinic phase, accompanied by the formation of microcracks. It is also well known that water vapor in the ambient gas promotes this aging process. Despite there are still some controversies about the detailed procedure and the exact role of the water vapor, it is generally agreed that the presence of water vapor is one necessary condition for the aging process. Therefore, the aging phenomenon might be suppressed if the TZP is not allowed to be directly contacted by water vapor, by the formation of some protective layer on the material surface. It has now been found that if a silica layer is formed on the surface of a TZP ceramic by a similar procedure, an improvement in the strength of the material is obtained. Furthermore, if the surface layer is effective in inhibiting the direct contact by water vapor with the material surface, the aging phenomenon can be suppressed.